I've seen it before -- people either get in such a rush to move or so sick of packing that they just throw things in boxes, without giving any thought to how that container will travel. Of course, the same people show a lot more concern about the packing job when they find their treasures damaged on the other end!
First off, try not to exceed 20 KILOS in each packed container -- otherwise they will be impossible to carry and your belongings might be crushed. As you pack, place heavier items on the bottom of each box and lighter items on top. Your containers will be more stable and less likely to tip over in transit.
Use strong TWINE or threaded packing tape to thoroughly seal your containers -- don't just fold the flaps in over themselves, unless you want everything spilling out as the truck is unloaded.